errors in most cases fall within the expected errors shown in Table IV. Elements other than platinum, palladium, rhodium, and iridium may be present in solutions studied, because iron, copper, nickel, zinc, silver, gold, arsenic, and antimony occur in platinum ores. Interferences may be of three types: enhancement of lines, absorption of lines, or proximity of an interfering line to a line studied. Because of the low concentrations in liquids, absorption and enhancement effects are small, as the atoms are far apart. Silver with a Kp line a t 0.497 A. could enhance the palladium K lines, since the K absorption edge for palladium occurs a t 0.5079 A. (1). I n order to determine whether silver causes enhancement, a solution mas prepared containing 20 mg. per ml. of silver and 0.62 mg. per ml. of palladium. Another solution contained 20 mg. per ml. of silver. Both were counted a t the palladium Ku line a t 16.76" 20. Results shown in Table V are within the experimental error of the method and indicate that no enhancement occurs.
Interference may also be caused by the proximity of the silver K , line a t Table VI. Sensitivities for Platinum, 16.01O to the palladium K,line at 16.76", Palladium, Rhodium, and Iridium by and by the proximity of the gold L B ~ X-Ray Fluorescence Method and Lp, lines a t 31.88" and 32.02", Sensitivity, respectively, to the platinum Lp, line Element Line Mg./Ml. at 32.29'. Pt LB1 0.10 Pd K, 0.20 CONCLUSIONS Rh K, 0.14 Ir LU, 0.02 X-ray fluorescence affords a rapid method for determining platinum, palladium, rhodium, and iridium in solutions. The per cent errors shown in but decrease sensitivity for platinum Table I V were calculated on an average and iridium. error in counting for one standard deviation of 1.86yG. LITERATURE CITED Sensitivities for the four elements (1) Compton, -4.H., Allison, S. K., ''Xstudied are shown in Table VI. SensiRays in Theory and Ex eriment,!' tivity is the concentration giving a count Van Nostrand, New Yorf, 1954. of 1Oyoabove background. Sensitivity ( 2 ) Davis. E. N.. Hoeck. B. C.. ANAL. -for iridium and platinum may be inCHEW.27, iSS0 (1955). ' (3) Gilchrist, R., Wichers, E., J . Am. creased by use of a 100-kv. x-ray tube Chem. SOC.57, 2565 (1935). which could excite their K spectra. (4) Pfeiffer. H. G.. Zemanv, P. D., Suture Rhodium and palladium sensitivities are 174, 397 (1954). poor, owing to low sensitivity of the RECEIVED for review October 18, 1956. argon-filled Geiger tube in this region. Accepted February 23, 1957. Much of A krypton-filled Geiger tube mould inthe work was carried out under a grant crease sensitivity for these tn-o elements from the Sltional Science Foundation. \-I
\
~
,
Determination of Normal Paraffins in Olefin-Free Petroleum Distillates by Molecular Sieve Sorption and Refractometry R. D. SCHWARTZ and D. J. BRASSEAUX Explorafion and Production Research Division, Shell Development Co., Houston, Tex.
b Heptane, octane, nonane, and decane can be determined by a refractometric method in olefin-free petroleum distillates. The sample of petroleum is first distilled to yield fractions containing only one n-paraffin. These distillates are then fractionated on chromatographic columns packed with Linde Type 5 A Molecular Sieve, which selectively removes the n-paraffin. The refractive index of a portion of the n-paraffin-free eluate is measured. This value, the refractive index of the original distillate, and the literature value for the refractive index of the pure n-paraffin assumed to b e present in the distillate are used to compute the n-paraffin content of the fraction. The results show an average precision of better than 1% of the n-paraffin content and an average error of less than 2y0 of the n-paraffin content.
1022
ANALYTICAL CHEMISTRY
SEW
synthetic zeolite, Linde Type
5A Molecular Sieve, has recently become available from the Linde iiir Products Co. This sorbent, with uniform pores close to 5 A. in diameter, has the ability to sorb n-paraffins from hydrocarbon mixtures 14). The object of this investigation was to develop a method using this sorbent for the determination of heptane, octane, nonane, and decane in olefin-free petroleum distillates. It appeared that this technique might supplement precison fractional distillation ( 5 ) , which is useful for the determination of the C1 to C5 n-paraffins, and urea adduct techniques (6), which are best suited for the determination of dodecane and higher boiling n-paraffins. Infrared (Z), mass spectrometric ( 8 ) , and gas chromatographic procedures ( I ) have been proposed for the determination of nparaffins, but these techniques require
rather complex equipment and careful calibration. APPARATUS
Distillation column, 1 inch in diameter, 3 feet in length, containing HeliPak packing, Podbielniak Inc., Chicago,
111.
Separatory funnels, Fisher and Porter No. 31-1725, with Teflon plugs Vibrator. A Vibro-Graver engraver (Burgess T'ibrocrafters, Inc., Chicago, Ill.) fitted with a stopper over the engraver end for packing the chromatographic columns. Chromatographic tubes, Corning S o . 38460, 300 mm. in length. I n some cases, the upper section was replaced with a 600-mm. section. The lower sections of the tubes were tapered t o permit the collection of eluate fractions in 2-ml. vials. A column, 19 mm. in inside diameter by 550 mm. long, was used to prepare the dearomatized distilla t e.
Refractometer. A Bauacli ti Lomb Abbe-56 with a sodium D light source !vas used to measure refract&e indices at 25.0' C. REAGENTS
Hrytane, 2,2,4-trimethylpentaneq methylcyclohexane, and toluene were Phillips Pure Grade having a guaranterd minimum purity of 99 mole %. Decane, 3-methylheptane, and octane were' Humphrey-Wilkinson products having a minimum purity of 95 mole 70. Soltrol 130 and Soltrol 170, isoparaffinic mixtures free of n-paraffins, were obtained from Phillips Petroleum Co.. SDecial Products Dirision. Bartlesville, Okla. Linde Type cih hIolecular Sieve. For the develomnent of the method. '/,,-inch pellets-mre used; 14- to 30: mesh material m s used for the procedure. Davison silica gel, 28 to 200 mesh. PROCEDURE
Preparation of Distillates. Distill a thoroughly dried sample of crude petroleum under conditions providing a separating efficiency of a t least 10 theoretical plates. The distillation column described under Spparatus is satisfactory. Distill a 2-liter charge at a 12 to 1 reflux ratio. Collect the fiactions boiling from 84' to 112', 112' to 138', 138' t o 162', and 162' t o 18.5' c. Removal of ?+Paraffins from Distillates. Transfer approximately 30 nil. of distillate sample to a 60-ml. separatory funnel. Lubricate the standard taper joint of a clean, dry adsorption column (10 X 600 mm.) with a minimum amount of lubricant. Pack the column with approximately 34 grams of Rlolecular Sieve, 14 t o 30 mesh, using a vibrator t o ensure proper packing. Add the sample to the packed colunin from the separatory funnel, adjusting the rate to about 10 drops per minute. Collect 1-ml. portions of the eluate in small numbered vials which liave been marked to contain approximately 1 ml. of liquid. It will generally be possible t o collect a t least 8 ml. of eluate. Seal the vials as the eluate fractions are collected. Each container of Molecular Sieve should be tested for its ability to remove n-heptane from a mixture of 30% by volunw n-heptane and 70% by rolume methylcyclohexane. When a 40-ml. sample is passed at a rate of about 10 drops per minute through a column packed as described above, a t least 10 1-nil. cluatc samples free of n-heptane should be obtained. These samples should be collected and analyzed by measurement of refractive index. The Molecular Sieve is extremely hygroscopic, and expowre to the atmosphere must he kept to a minimum. Determination of n-Paraffin Content. Measure the refractive index of each sample a t 25.0" C., using the sodium D spectral line and a suitable refractometer. Compute the n-paraffin content of the distillate fractions using the equation
5 11-paraffins( b y voliime'r
COLUMN TESTS WITH SORBENT
=
b-c ?b
x 100
(1)
n-liere
a = 1.3851, 1.3951. 1.4031, and 1.4097 respectively,' for the'84'to 112' 112' t o 138'. 138' to 162'. and 162' to 185d C. distillates ' b = refractive index of an eluate sample free of n-paraffins (The refractive index of the n-paraffinfree eluate which should be used in these calculations is the highest refractive index value obtained for the eluate portions of any single distillate sample, Although the first eluate portions are free of n-paraffins, they usually have a refractive index lower than subsequent portions. This effect is due to some sorption of aromatic compounds by the Molecular Sieve.) c = refractive index of original distillate BATCH TEST SORBENT
The Type 5A sorbent was available initially as a very fine powder, 0.5 to 5 microns in diameter, and as cylindrical pellets 1/8 or 1/16 inch in diameter. These particle sizes are not suitable for efficient chromatographic adsorption with liquid samples. Therefore, a series of batch experiments was performed with the 1/16-inch pellets. Mixtures containing 'heptane, octane, or decane with typical isoparaffin, cycloparaffin, or aromatic hydrocarbons were prepared. Portions of 5 ml. of each of these mixtures were treated with 6 grams of the pellets. The samples were shaken for 20 minutes on a mechanical shaker and allowed to stand for 3 hours. A portion of the unsorbed liquid r a s analyzed by refractive index measurement. The results obtained indicate that n-heptane, octane, and decane are preferentially sorbed from these mixtures (Table I).
Table l.
The next phase of this investigation vas to evaluate a sample of 14- to 30mesh sorbent by a column technique. It would be desirable to use an even finer adsorbent, but it is not being produced a t this time. It is, of course, possible t o crush pellets or 14- t o 30mesh sorbent to a smaller size. But because of the extremely hygroscopic nature of this material, such crushing must be done in a dry box or subsequent reactivation xi11 be required to remove sorbed water. A series of eight synthetic hydrocarbon mixtures was tested. Each sample mas added to a chromatographic column, 10 x 300 mm., packed with li grams of Type 5-4 sorbent. The frontal analysis technique was used in all cases and eluate fractions were collected by gravity flow s-ithout the use of pressure or vacuum, I n the first three cases small portions of the eluate were weighed. For the next five mixtures small measured volumes of eluate were collected. The refractive index of all eluate portions was determined a t 25.0' C. The results obtained are presented in Table 11. Because the refractive index of the first portions of eluate obtained for mixtures A , B , and C corresponds to the value for methylcyclohexane, the results indicate that the column technique permits recovery of n-paraffinfree eluate from these binary mixtures. The results for mixture D could not be easily explained a t first, as no inflection point was expected in the frontal analysis curve until the nparaffins began to break through. Mixtures E , F,G, and H were analyzed in order to determine the cause of the first inflection point obtained with mixture D. The inflection for mixture G is due to the sorption of toluene. The small change in toluene content for the 5.0 nil. of eluate from which tol-
Batch Test Data with Molecular Sieve Type 5 A
Mixturen Volume
Refractive Index (nz;) Unsorbed Mixture liquid 1 394; 1 3962
% Conclusion Heptane 15 Heptane sorbed 3-1Iethylheptane 85 Heptane 15 1.4154 1 4200 Heptane sorbed Rfethylcyclohesane 85 Heptane 15 1,4763 1.4937 Heptane sorbed Toluene 85 Octane 15 1.4772 1.493T Octane Eorhed Toluene 85 Decane 15 1.3922 1.3890 Decane sorbed 2,2,4Trimethylpentane 85 a BPI refractive index dat'a (naj) for high purity hydrocarbons ( 7 ) : heptane, 1.3851; octane, 1.3951; decane, 1.4097; 3-methylheptane, 1.3961 : toluene, 1.4941: 2,2,4trimethylpentme, 1.3890: met,hylcyclohexane,1.4206. ~~~
VOL. 29, NO. 7 , JULY 1957
~~
1023
uene was sorbed in this test indicate? that the capacity of the sorbent for toluene under these conditions is approximately 0.05 ml. per 17 grams. TWOdistillate blends from an East Texas Crude were analyzed by the frontal analysis technique described above in order t o compare the elution curves to that obtained for mixture D. The results obtained (Table 111) indicate that distillate fractions yield elution curl-es similar to the curve for synthetic mixture D. The plateaus hetween 3.5 and 12.0 ml. and between 2.5 and 9.0 nil. for these samples repreqent the refractive index of eluate d i i c h is free of n-paraffinq and nhich lins the same aromatic content d s the original distillates. Another East Texas distillate (85' to 114" C.) I\ as dearomatized nith Dal-ison .ilica gel. The dearomatized material n a s added to a column containing Type 5,4 sorbent. Frontal analysis of the aromatic-free sample yielded an elution curve without the initial inflection due to sorption of arom at 1cs. ' Tlie results obtained for the dearomatization and the subsequent treatment with Type 5.1 sorbent are prewnted in Table IV. n-PARAFFIN CONTENT BY REFRACTIVE INDEX MEASUREMENTS
The refractive index of ideal hydrocnrhon mixtures is additive on a volume h i s ( 3 ) . Binary test mixtures, such aq those used for the calibration of distillation equipment, may be analyzed rapidly and accurately by refractive index measurement. Ordinarily, petroleum distillates which have not been cracked contain no olefins. The boiling points of the n-paraffins ( 7 ) are such that fractional distillation permits the preparation of fractions containing only one n-paraffin. Thus, fractions lvith boiling ranges of 84" t o 112') 112' to 138", 138" to 162") and 162' to 185" C. 3hould contain heptane, octane, nonane, and decane, respectively. Each of these distillates can be considered as a tivocomponent system-the n-paraffin portion and the remainder. The n-paraffin content can be calculated from the refractive indices of the distillate, a portion of the n-paraffin-free eluate, and the n-paraffin present in the fraction. Tlie equation employed has the form
n-here
X
per cent n-paraffins in distillate by volume a = refractive index of n-paraffin assumed to be in distillate b = refractive index of an eluatc sample free of n-paraffins c = refractive index of original distillate =
1024
ANALYTICAL CHEMISTRY
Table 11. ____
Column Data for Synthetic Blends
AIixtiire A
B _ _ _ - _ _ ~- ~ Llisture ~ - ~ H!-drucnrbon % n 'd" n-Heptniic 50 1.1030 Methylcyclotiesarie 50 SFlow rat,e o i sample introduction, 10 drops per minute Total Keight of Eluate, Grams n 2Dj 1.4208 0.19 ~~~
Hydrocarbon n I 1 'D' n-Heptane 70 I 3'wJ Methylcyclohexane 30 Flow rate of sample introduction, 1 S C ~ per minute Total Weight of Eluate, Grams n -5 0 19 1 1208 0 40 1 4208 0 57 1 4104 1 OCI 1 4070 1 4001 1.53 2 30 1 3970
O ~
0.71 1 33 1 75 2 01 2 24 2 98 3 64
1 4175
1,4058
1 4040
Mixture C Hydrocarbon 70 It n-Heptane 30 1 4100 Methylcyclohexane 70 Flow rate of sample introduction, 5 drops per minute Total Keight of Eluate, Grams i~?; 0.21 1 4208 1 4208 1.09 2 08 3 03 4 21 4 94 1 4208 5 26 1 4190 5 48 1.4105 6 34 1 4100 6 73
Mixture D Hydrocarbon 70 n' 2 n-Hrptane 50 1 4135 11ethrlc yclohexane 20 2,2,4Trimethylpentane 20 Toluene 10 Flox rate of sample introduction, 18 drops per minute Total \-ohme of Eluate,
Mixture E
Mixture F Hydrocarbon mc ny n-Heptane 50 1 38i2 2,2,4-Trimeth>lpentane 50 Flow rate of sample introduction, 16 drops per minute Total T'olrnne of Eluate, n2?
- _ _ _ ~
2;
Hydrocarbon n-Heptane Toluene
%
50
n25
1 4380
50
Flow rate of sample introduction, 16 drops per minute Total Volume of Eluate, ,t2 M1. ' 1.4935 0.5 1 4935 1.0 1.4935 1.5 7.4934 2.0 1 ,4838 3.0 1 .4522 4.0 1 4418 5.0 5
Mixture G Hydrocarbon % iL 'D" 2,2,4Trimethylpentane 70 1.4198 Toluene 30 Flow rate of sample introduction, 18 drops per minute Total Volumr of Eluate, RI1. n 's 1 4180 0 5 1 4182 1 0 1 4185 1 5 1 4189 2 0 1 4192 30 1 4195 40 1 4195 5.0 1 4198 6 0 1 4198 7 0
111.
ik';
0 5 i_n
1 5 20 2 5 3 0
3 5 4 0 4 5
.
1 ,4200 1 4205
1 4209
1 4211 1 4212
1.4210
1 4100
1,4150 1,4101
-
Ill. 0 5
1 0 1 5
1,3890 1,3890
1.3890
2 0 2 5
30
4.0 -5 0 0 0
1 3872
Mixture H Hydrocarbon % n 2' 2,2,PTrimethylpentane 50 1 4051 Methylcyclohexane 50 Flow rate of sample introduction, 1s drops per minute Total Volume of Eluate, 1.11.
0.5 I .O
2.0 3.0
71 2;
1,4050 1.4051 1.4051 1,4051
Table 111.
Column Data for East Texas Distillates
85" to 114" C., n2Dj = 1.4090 1210~mtc of sample introduction, 15 drops per minute -~ Total volume of eliiate,
nil.
ny
0 5
1 4123 1 4125 1 4128 1 4130 1 4132 1,4131 1.4138 1,4138 1.4135 1.4130 1.4121 1,4110 1,4102 1,4098 1.4092 1,4090 1 ,4090 1.4000
114" to 156' C., ny = 1.4238 F l o rate ~ of sample introduction, 30 drops per minute Total volume of eluate, nil. n25 D
Table VI. Determination of n-Octane and n-Decane by Refractive Index Measurements
Sample KO.
16 17
1 0
1
